1. Field of the Invention
The present invention relates to polyisocyanate mixtures that are soluble in non-polar solvents, to a process for their production and to their use as starting components for the production of polyurethane plastics, which are highly dilutable with non-polar solvents, in particular, as crosslinking agents for lacquer binders and/or binder components having isocyanate-reactive groups.
2. Description of the Prior Art
Polyisocyanates having aliphatically or cycloaliphatically bound isocyanate groups may be used as high-grade, non-yellowing two-component (2K) polyurethane (PU) coating compositions in many areas of application, for example in automotive coatings or in the construction sector. Due to their relatively high viscosity, these (so-called light-fast) lacquer polyisocyanates are generally used in a form diluted in solvents.
Since it is desired to reduce emissions of toxicologically or environmentally questionable solvents from lacquers and coatings, repeated attempts have been made in recent years to replace these solvents with alternative products, for example with non-polar, environmentally compatible solvents, such as mineral spirits or turpentine. It has not previously been possible to dilute 2K PU coating compositions with these non-polar solvents because they are not sufficiently compatible with the known aliphatic or cycloaliphatic polyisocyanate crosslinking agents.
The aliphatic or cycloaliphatic polyisocyanates, which contain biuret, isocyanurate and/or uretidione groups, are prepared from commercially available diisocyanates, such as hexamethylene diisocyanate (HDI) and/or 4,4xe2x80x2-diisocyanatodicyclohexylmethane, and are particularly important for the production of light-fast polyurethane coatings, are all only sparingly soluble non-polar organic solvents.
In contrast, allophanate-modified polyisocyanurates are much more readily soluble in non-polar solvents. These polyisocyanates may be obtained according to EP-A 238,013, EP-A 496,208, EP-A 524,500, EP-A 524,501, EP-A 566,037, U.S. Pat. No. 4,789,705, U.S. Pat. No. 5,086,175, U.S. Pat. No. 5,258,482, U.S. Pat. No. 5,290,902, U.S. Pat. No. 5,380,792, JP 0 403 621 8 A, JP 0 413 012 4 A, JP 0,413 017 1 A, JP 6 107 201 3 A or JP 901 266 0 A by reacting aliphatic and/or cycloaliphatic diisocyanates with linear or branched aliphatic or cycloaliphatic monoalcohols or diols in the presence of trimerization catalysts. However, when producing such allophanate trimers from monomeric diisocyanates, the reaction becomes exothermic after an unpredictable latency period. When using batch methods, it is thus necessary to dissipate the resultant heat of reaction, which is not always simple to achieve. Moreover, the allophanate-modified polyisocyanurates described in the stated publications, in particular those based on monoalcohols, give rise to coatings having considerably poorer resistance to solvents and chemicals than corresponding unmodified polyisocyanurates.
Pure allophanate polyisocyanates or so-called heteroallophanates, which are described, for example, in EP-A 807,623, also have good solubility in non-polar solvents. However, these products also result in coatings having only moderate resistance characteristics and, thus, are not well suited as crosslinking components for high-grade coating compositions. This also applies to urethanes that are prepared from di- or polyisocyanates and long-chain monoalcohols or dimer diol, even though according to the teachings of U.S. Pat. No. 5,086,175, JP 0 413 278 2 A, JP 0 423 065 1 A and JP 0 726 805 9 A, they exhibit good compatibility with hydrocarbons.
Trimers of special diisocyanates having lipophilic residues, for example cycloaliphatic diisocyanates having a long-chain aliphatic residue attached to the cycloaliphatic ring (EP-A 818,485), branched linear aliphatic diisocyanates (DE-A 3,151,855) or saturated aliphatic or cycloaliphatic diisocyanates having 11-20 carbon atoms (JP 0 324 462 3 A) have also been suggested as polyisocyanate crosslinking agents dilutable with non-polar solvents. However, these special diisocyanates are not available on a large industrial scale and are thus costly.
DE-A 3,613,064, U.S. Pat. No. 3,304,286 and JP 0 230 247 9 A describe isocyanate-functional prepolymers, which are prepared from polyisocyanates and polyols and are readily soluble in non-polar solvents. These products frequently have only a low content of free isocyanate groups and a low functionality. They are consequently less suitable as a crosslinking component for 2K PU systems, but are instead preferably used as the sole binder in moisture-curing one-component systems.
No light-fast polyisocyanates are known, which exhibit good solubility in non-polar solvents and meet all property requirements, i.e. in particular result in highly crosslinked coatings resistant to solvents and chemicals, and which may be obtained in a simple, low-cost process.
An object of the present invention is to provide novel polyisocyanate mixtures having aliphatically and/or cycloaliphatically-bound isocyanate groups and do not suffer from the disadvantages of the prior art.
This object may be achieved with the polyisocyanate mixtures according to the invention, which are described in greater detail below along with a process for their production. The polyisocyanate mixtures according to the invention are based on the surprising observation that, by reacting aliphatic and/or cycloaliphatic polyisocyanates having a low monomer content with alcohols under allophanatization conditions, it is possible to produce highly functional polyisocyanate mixtures which are readily soluble in non-polar solvents even at low temperatures and which, in combination with hydroxy-functional lacquer binders, result in coatings distinguished by outstanding resistance characteristics.
Although some publications relating to the production of polyisocyanates containing allophanate groups, for example EP-A 000,194, EP-A 303,150 and EP-A 682,012, do make general mention of polyisocyanates having a functionality of  greater than 2, such as trimerization products of HDI or IPDI, within long lists of suitable starting polyisocyanates, it is not taught or suggested in any of the above-stated publications that reaction products of polyisocyanates having low monomer contents with alcohols produced under allophanatization conditions exhibit particularly good solubility characteristics in non-polar solvents.
The present invention relates to polyisocyanate mixtures prepared from aliphatic and/or cycloaliphatic diisocyanates and having
a) an average isocyanate functionality of at least 2.0,
b) a content of isocyanate groups (calculated as NCO; molecular weight=42) of 4.0 to 28.0 wt. % and
c) a content of alkoxy groups having up to 36 carbon atoms of 2.0 to 37.5 wt. %,
wherein the alkoxy groups are constituents of allophanate groups or optionally urethane groups, provided that  greater than 50 mol % of the alkoxy groups are a constituent of allophanate groups that are connected to at least two polyisocyanate molecules which are each prepared from at least two diisocyanates.
The present invention also relates to a process for preparing these polyisocyanate mixtures by reacting
A) a polyisocyanate component having an average NCO functionality of 2.0 to 5.0, a content of aliphatically and/or cycloaliphatically bound isocyanate groups (calculated as NCO; molecular weight=42) of 8.0 to 27.0 wt. % and a content of monomeric diisocyanates of less than 1 wt. % with
B) (cyclo)aliphatic alcohols having up to 36 carbon atoms,
at an NCO/OH equivalent ratio of 1.9:1 to 80:1 such that  greater than 50% of the urethane groups formed by the NCO/OH reaction are converted to allophanate groups.
The present invention further relates to coating compositions, which are highly diluted or dilutable with non-polar solvents and contain these polyisocyanate mixtures and lacquer binders or binder components optionally containing isocyanate-reactive groups.
Polyisocyanate component A) has an average NCO functionality of 2.0 to 5.0, preferably of 2.3 to 4.5; a content of isocyanate groups of 8.0 to 27.0 wt. %, preferably of 14.0 to 24.0 wt. %; and a content of monomeric diisocyanates of less than 1.0 wt. %, preferably less than 0.5 wt. % and more preferably 0.0001 to 0.5 wt. %. Component A) contains one or more organic polyisocyanates having aliphatically and/or cycloaliphatically bound isocyanate groups.
Polyisocyanate component A) is selected from any desired polyisocyanates built up from at least two diisocyanate molecules and prepared by modifying monomeric, aliphatic and/or cycloaliphatic diisocyanates to incorporate uretidione, isocyanurate, allophanate, biuret, iminooxadiazinedione and/or oxadiazinetrione groups. Their preparation is described, for example, in J. Prakt. Chem. 336 (1994) 185-200, DE-A 1,670,666, DE-A 1,954,093, DE-A 2,452,532, DE-A 3,700,209 and DE-A 3,900,053 or EP-A 336,205, EP-A 339,396 and EP-A 798,299.
Suitable diisocyanates for preparing polyisocyanates A) are those having a molecular weight of 140 to 400 and having aliphatically and/or cycloaliphatically bound isocyanate groups. Examples include 1,4-diisocyanatobutane, 1,6-diisocyanatohexane (HDI), 2-methyl-1,5-diisocyanatopentane, 1,5-diisocyanato-2,2-dimethylpentane, 2,2,4- or 2,4,4-trimethyl-1,6-diisocyanatohexane, 1,10-diisocyanatodecane, 2,4- or 2,6-diisocyanato-1-methylcyclohexane, 1,3- and 1,4-diisocyanatohexane, 1,3- and 1,4-bis(isocyanatomethyl)-cyclohexane, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl-cyclohexane (isophorone diisocyanate or IPDI), 4,4xe2x80x2-diisocyanatodicycylohexylmethane, 1-isocyanato-1-methyl-4(3)isocyanatomethylcyclohexane, bis(isocyanato-methyl)norbornane and mixtures thereof.
Preferred starting components A) are polyisocyanate mixtures containing isocyanurate and/or iminooxadiazinedione groups and prepared from HDI, IPDI and/or 4,4xe2x80x2-diisocyanatodicyclohexylmethane.
Component B) is selected from saturated or unsaturated, linear or branched alcohols. Examples include monoalcohols containing 1 to 36, preferably 1 to 23 carbon atoms, such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert. -butanol, n-pentanol, 2-hydroxypentane, 3-hydroxypentane, the isomeric methylbutyl alcohols, the isomeric dimethylpropyl alcohols, n-hexanol, n-heptanol, n-octanol, n-nonanol, 2-ethylhexanol, trimethylhexanol, n-decanol, n-undecanol, n-dodecanol (lauryl alcohol), n-tetradecanol, n-pentadecanol, n-hexadecanol, n-heptadecanol, n-octadecanol (stearyl alcohol), 2,6,8-trimethylnonanol, 2-tert.-butylcyclohexanol, 4-cyclohexyl-1-butanol and the linear or branched primary alcohols sold by Henkel AG under the trade name Lorol.
Also suitable as component B) are dialcohols and/or higher functional alcohols having n to 36, preferably n to 23 carbon atoms (where n=OH functionality of the alcohol), such as 1,2-ethanediol, 1,2- and 1,3-propanediol, 1,2- and 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, 4,4xe2x80x2-(1-methylethylidene)biscyclohexanol, the isomeric butanediols, pentanediols, hexanediols, heptanediols, octanediols, nonanediols, decanediols and undecanediols; higher molecular weight diols such as 1,12-octadecanediol; and higher functional alcohols such as 1,2,3-propanetriol, 1,1,1-trimethylolethane, 1,2,6-hexanetriol, 1,1,1-trimethylolpropane, 2,2-bis(hydroxymethyl)-1,3-propanediol and 1,3,5-tris(2-hydroxyethyl) isocyanurate.
Other suitable alcohols B), which less preferred for the present invention, are those which contain, in addition to the hydroxyl group, other functional groups which are non-reactive towards isocyanate groups, such as ester groups, ether groups and other heteroatoms, e.g., halogen atoms, silicon or nitrogen.
Preferred components B) are monoalcohols and mixtures of these monoalcohols with dialcohols and/or higher functional alcohols.
The process according to the invention is performed by reacting starting components A) and B) with each other at temperatures of 40 to 180xc2x0 C., preferably of 50 to 150xc2x0 C., and at an NCO/OH equivalent ratio of 1.9:1 to 80:1, preferably of 2:1 to 50:1, such that  greater than 50 mol %, preferably at least 80 mol % and more preferably at least 90 mol %, of the urethane groups initially formed by the NCO/OH reaction react further to form allophanate groups.
The allophanatization reaction may be accelerated by optionally using suitable catalysts, such as the known allophanatization catalysts. Examples include metal carboxylates, metal chelates and tertiary amines, such as those disclosed in British patent GB 994,890; alkylating agents such as those disclosed in US patent U.S. Pat. No. 3,769,318; and strong acids, such as those disclosed in EP-A 000,194.
Suitable allophanatization catalysts include metal compounds such as zinc compounds (for example zinc(II) stearate, zinc(II) n-octanoate, zinc(II) 2-ethyl-1-hexanoate, zinc(II) naphthenate and zinc(II) acetylacetonate), tin compounds (for example tin(II) n-octanoate, tin(II) 2-ethyl-1-hexanoate, tin(II) laurate, dibutyltin oxide, dibutyltin dichloride, dibutyltin diacetate, dibutyltin dilaurate, dibutyltin dimaleate and dioctyltin diacetate), aluminum tri(ethylacetoacetate), iron(III) chloride, potassium octoate, compounds of bismuth, manganese, cobalt or nickel together with strong acids, such as trifluoroacetic acid, sulfuric acid, hydrogen chloride, hydrogen bromide, phosphoric acid or perchloric acid, and mixtures of these catalysts.
Catalysts, which are suitable although less preferred for the process according to the invention, include compounds which catalyze not only the allophanatization reaction but also the trimerization of isocyanate groups to form isocyanurate structures. Examples of these catalysts are described, for example, in EP-A 649,866, page 4, line 7 to page 5, line 15.
Preferred catalysts for the process according to the invention are zinc(II) compounds and/or bismuth(III) compounds. Especially preferred are zinc(II) n-octanoate, zinc(II) 2-ethyl-1-hexanoate, zinc(II) stearate and/or bismuth(III) 2-ethyl-1-hexanoate.
The catalysts are optionally used in the process according to the invention in a quantity of 0.001 to 5.0 wt. %, preferably of 0.005 to 1.0 wt. %, based on the total weight of the reactants.
The catalysts may be added to the reaction mixture using any desired method. For example, it is possible to incorporate the optional catalyst either into polyisocyanate component A) and/or into alcohol component B) before the actual reaction begins. It is also possible to add the catalyst to the reaction mixture at any desired time during the urethanization reaction or, in the case of a two-stage reaction, after urethanization, i.e. once the NCO content theoretically corresponding to complete conversion of isocyanate and hydroxyl groups to urethane groups has been reached. It is also possible to initially react one or more constituents of polyisocyanate component A) with alcohol B) in a urethanization reaction and then after urethanization to add the catalyst with the remaining constituents to polyisocyanate component A).
The course of the reaction may be monitored, for example, by titrimetric determination of the NCO content. Once the desired NCO content has been reached, i.e., once the molar ratio of allophanate groups to urethane groups in the reaction mixture is  greater than 1:1, preferably at least 4:1 and more preferably at least 9:1, the reaction is terminated. When the reaction is controlled purely thermally, this may be achieved, for example, by cooling the reaction mixture to room temperature. When, as preferred, an allophanatization catalyst is used, the reaction may be terminated by adding suitable catalyst poisons, for example, acid chlorides such as benzoyl chloride or isophthaloyl dichloride. However, such termination is not essential in the process according to the invention.
In the process according to the invention, the nature and quantities of the starting components are selected within the stated parameters such that the resultant polyisocyanate mixtures comply with the requirements set forth above in a) to c), wherein a) the average NCO functionality is at least 2.0, preferably 2.3 to 9.9 and more preferably to 2.8 to 5.8; b) the NCO content is 4.0 to 28.0 wt. %, preferably 5.0 to 24.0 wt. % and more preferably to 8.0 to 21.0 wt. %; and c) the content of alkoxy groups is 2.0 to 37.5 wt. %, preferably 3.0 to 29.0 wt. % and more preferably 4.0 to 20.0 wt. %., wherein the alkoxy groups are a constituent of an allophanate group or optionally a urethane group, provided that  greater than 50 mol %, preferably at least 80 mol % and more preferably at least 90 mol %, of the alkoxy groups are a constituent of allophanate groups that are connected to at least two polyisocyanate molecules which are each prepared from at least two diisocyanates.
The content of alkoxy groups is determined as follows accordingly to formula [1]:                               Percentage          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          alkoxy          ⁢                      xe2x80x83                    ⁢          groups                =                                                            Weight                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                Alcohols                ⁢                                  xe2x80x83                                ⁢                B                            )                        xc3x97            100                                                                                Weight                  ⁢                                      xe2x80x83                                    ⁢                  of                  ⁢                                      xe2x80x83                                    ⁢                  polyisocyanates                  ⁢                                      xe2x80x83                                    ⁢                  A                                )                            +                              Weight                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                alcohols                ⁢                                  xe2x80x83                                ⁢                B                                      )                                              [        1        ]            
The NCO functionality of the products according to the invention may be calculated from the nature and functionality of the starting components in accordance with formula [2]                    F        =                                            ∑                              xe2x80x83                            ⁢                              equiv                ⁢                                  xe2x80x83                                ⁢                NCO                                      -                          ∑                              xe2x80x83                            ⁢                                                                    (                                          1                      +                      x                                        )                                    ·                  equiv                                ⁢                                  xe2x80x83                                ⁢                OH                                                                        ∑                              xe2x80x83                            ⁢                              (                                                      equiv                    ⁢                                          xe2x80x83                                        ⁢                    NCO                                                        f                    NCO                                                  )                                      +                          xe2x80x83                        ⁢                          (                                                equiv                  ⁢                                      xe2x80x83                                    ⁢                  OH                                                  f                  OH                                            )                        -                          ∑                              xe2x80x83                            ⁢                                                                    (                                          1                      +                      x                                        )                                    ·                  equiv                                ⁢                                  xe2x80x83                                ⁢                OH                                                                        [        2        ]            
wherein 1 xe2x89xa7xxe2x89xa70 and xe2x80x9cxxe2x80x9d means the proportion of the urethane groups converted into allophanate groups in the process according to the invention. The functionality fNCO of the starting polyisocyanates A) may be calculated from the NCO content and the molecular weight, and the molecular weight may be determined by gel permeation chromatography (GPC) or vapor pressure osmosis. fOH is obtained from the number of OH groups in the alcohols used or, in the case of the mixtures, from the amounts and the number of OH groups in the individual alcohols of the mixture.
The process according to the invention may optionally be performed in a suitable solvent which is inert towards isocyanate groups. Examples include known lacquer solvents such as ethyl acetate, 1-methoxy-2-propyl acetate, 3-methoxy-n-butyl acetate, acetone, 2-butanone, 4-methyl-2-pentanone, cyclohexanone, toluene, xylene, chlorobenzene, mineral spirits, substituted aromatics (for example those commercially available under the names Solvent Naphtha, Solvesso, Shellsol, Isopar, Nappar and Diasol), carbonic acid esters (such as dimethyl carbonate, diethyl carbonate, 1,2-ethylene carbonate and 1,2-propylene carbonate), lactones (such as xcex2-propiolactone, xcex3-butyrolactone, xcex5-caprolactone and xcex5-methylcaprolactone), propylene glycol diacetate, diethylene glycol dimethyl ether, dipropylene glycol dimethyl ether, diethylene glycol ethyl and butyl ether acetate, N-methylpyrrolidone, N-methylcaprolactam, and mixtures thereof.
The products according to the invention are clear, virtually colorless polyisocyanate mixtures which, unlike starting polyisocyanates A), may be diluted with non-polar solvents without becoming turbid. Even when highly diluted, for example at a solids content of 30 wt. %, and even at low temperatures, for example at xe2x88x9250xc2x0 C. to +5xc2x0 C., the products are non-settling solutions.
The non-polar solvents include, for example, Swasol 310 (Maruzen Petrochemical Co. Ltd.; aniline point 43.5xc2x0 C.), Exxon Naphtha No. 6 (Exxon Chemical Japan; aniline point 43xc2x0 C.), Exxon Naphtha No. 5 (Exxon Chemical Japan, aniline point 55xc2x0 C.) and others as described, for example, in EP-A 566 037 (c.f p.5, lines 40-46) and U.S. Pat. No. 5,086,175 (c.f. page 4, lines 43-55), or any desired mixtures of these solvents.
Polyisocyanates that have not modified according to the invention, in particular lacquer polyisocyanates A), may optionally be added to the polyisocyanate mixtures according to the invention. The amounts are preferably selected such that the resulting polyisocyanates comply with the conditions stated above in a) to c) and consequently also comprise polyisocyanate mixtures according to the invention. The mixtures generally contain
(i) the allophanate-modified polyisocyanates according to the invention and
(ii) polyisocyanates that have not been modified according to the invention.
The polyisocyanate mixtures according to the invention are valuable starting materials for the production of polyurethane plastics using the isocyanate polyaddition process. They are outstandingly suitable as crosslinking components for 2K PU coating compositions, in which the polyhydroxyl compounds present as reaction partners for the polyisocyanates are known preferably polyether polyols, polyester polyols and/or polyacrylate polyols. Lacquer binders or lacquer binder components having isocyanate-reactive groups other than hydroxyl groups are also suitable as reaction partners for the polyisocyanate mixtures according to the invention.
These reaction partners include polyurethanes or polyurethanes which are crosslinkable by virtue of the active hydrogen atoms present in the urethane or urea groups. Other suitable reaction partners include secondary amines of the type described, for example, in EP-A 403,921 or polyamines having blocked amino groups, such as polyketimines, polyaldimines or oxazolones. In the presence of moisture either unblocked amino groups are formed or, in the case of oxazolones, hydroxyl groups are formed, which subsequently react with the polyisocyanate mixtures according to the invention. Minor amounts of non functional lacquer binders may also be added to the polyisocyanate mixtures according to the invention in order to achieve highly specific properties, for example, as additives to improve adhesion.
The polyisocyanate and/or binder components in two component coating compositions lacquers based on the polyisocyanate mixtures according to the invention are generally used in a form diluted with solvents. Suitable solvents include the solvents described above for the performance of the products according to the invention or any desired mixtures of such solvents. Preferably, however, in 2K PU coating compositions containing the polyisocyanate mixtures according to the invention, the reaction partners are used in the form of dilute solutions in non-polar solvents, more preferably in non-polar solvents presenting only a slight environmental hazard, such as Swasol 310 (Maruzen Petrochemical Co. Ltd.; aniline point 43.5xc2x0 C.), Exxon Naphtha No. 6 (Exxon Chemical Japan; aniline point 43xc2x0 C.), Exxon Naphtha No. 5 (Exxon Chemical Japan, aniline point 55xc2x0 C.) and others as described, e.g., in EP-A 566,037 (page 5, lines 40-46) and U.S. Pat. No. 5,086,175 (page 4, lines 43-55), and any desired mixtures of such solvents.
The polyisocyanate mixtures may also be used in a form blocked with the known blocking agents from polyurethane chemistry in combination with the lacquer binders or lacquer binder components as one-component PU stoving systems. Suitable blocking agents include malonic acid diethyl ester, acetoacetic ester, acetone oxime, butanone oxime, xcex5-caprolactam, 3,5-dimethylpyrazole, 1,2,4-triazole, dimethyl-1,2,4-triazole, imidazole and mixtures thereof.
When the polyisocyanate mixtures according to the invention are used as the crosslinking component for lacquer binders, they are generally used in quantities corresponding to an equivalent ratio of (optionally blocked) NCO groups to isocyanate-reactive groups, in particular alcoholic hydroxyl groups, of 0.5:1 to 2.0:1, preferably of 0.8:1 to 1.2:1 and more preferably of 0.9:1 to 1.1:1.
Any desired substrates may be coated with coating compositions formulated using the polyisocyanate mixtures according to the invention. Examples include metal, wood, glass, stone, ceramic materials, concrete, rigid and flexible plastics, textiles, leather and paper. These substrates may optionally be provided with known primers before coating.
The coating compositions may also contain known additives such as catalysts, leveling agents, coloring pigments, extenders or flatting agents. The resulting coatings generally exhibit good technical properties even when dried at room temperature. They may also be dried under forced conditions at elevated temperature or by stoving at temperatures of up to 260xc2x0 C.
The polyisocyanate mixtures according to the invention permit the formulation of high-grade, light-fast coating compositions which, in comparison with previously known solvent-based 2K PU systems, exhibit low or zero emissions of environmentally questionable solvents and, in comparison with coating compositions dissolved in conventional solvents, exhibit distinctly improved overcoating characteristics. The higher functionality of the polyisocyanate mixtures according to the invention in comparison with, for example, allophanate trimers, results in improved lacquer properties, such as resistance to solvents and chemicals.